The virtually unlimited optical bandwidth in fibers has motivated researchers and developers to shift certain network operations to the optical frequency domain for improving a system's performance and transmission capacity. Generally, this has been effected by utilizing wide band communications networks which allocate channels in the optical frequency domain. Moreover, these networks usually employ both wavelength division multiplexing and wavelength division switching for achieving multi-channel operation.
Wavelength filters, which connect and tune between channels, are one of the key devices required for the above stated networks. For example, in subcriber loop networks, all channels may be broadcast to every subscriber in the network, with the subscriber selecting the desired channel(s) via a tunable optical filter. As might be expected, several types of tunable wavelength filters have been proposed in the literature. For a review of wavelength tunable optical filters, see Kobrinski et al., IEEE Communications Magazine, pp. 53-63 (1989). In each of the proposed optical filters, the filtering mechanism may be viewed as a coupling mechanism between wave eigenmodes caused by some perturbative mechanism, e.g. resonant cavity, to effectuate the filtering action. Among the various filters proposed, such as electro-optic devices, acoustic-optic devices, Distributed Bragg Reflector devices, and Distributed Feedback devices, each may be described in the above manner.
It is well known that resonant cavities have a transmission characteristic with peaks and nulls in the frequency domain and that such a characteristic can be used advantageously for optical filtering. However, resonant cavities generally exhibit multiple transmission peaks that correspond to the longitudinal modes of the cavity which limit the tuning range since the mode spacing corresponds to the maximum range over which incoming signals can be spaced without overlapping. Resonant cavities, moreover, have usually been realized by using Fabry-Perot etalons which are difficult to integrate on a photonic integrated circuit. Because of these limitations, various new resonant structures have been designed for extending the tuning range and for achieving integratability. They include resonant laser structures that are biased below their lasing thresholds in their operation as resonant amplifiers. With respect to wavelength tunable optical filters, selected documents as discussed below are of interest.
U.S. Pat. No. 4,057,321 discloses a spectroscopically selective filter comprising two Bragg reflectors with Distributed Feedback disposed on a film waveguide in a tandem arrangement. In addition, applying a voltage via a pair of electrodes, arranged on opposite surfaces of the waveguide, and between the two reflectors, adjusts the optical path length between reflectors for achieving tunability in a manner analogous to a Fabry-Perot arrangement. While tunability is achieved, the filters exhibit a transmission peak of.about.6-10 .ANG. located between two reflective regions, known as the stopband. More importantly, outside the stopband the transmission is high and, as such, it limits the usable tuning range to the width of the stopband region, typically less than 800 GHz.
U.S. Pat. No. 4,750,801 discloses a grating resonator filter for achieving a filter bandwidth less than 1 Angstrom. The filter comprises first and second grating sections which are geometrically in phase with each other. That is, the distance between a grating peak in one section and any one grating peak in the second grating section is an integer number of grating periods. Furthermore, a phase section between the two grating sections, having a reduced refractive index, yields a .pi./2 (90.degree.) phase shift between the two grating sections. Similar approaches, but utilizing carrier injection have been used to achieve tunability with ranges of approximately 40-50 Angstroms. See, for example, T. Numai et al., Appl. Phys. Lett., Vol. 53, No. 2, pp. 83-85 (1988).
Numai et al., in Appl. Phys. Lett., Vol. 54., No. 19, pp. 1859-60 (1989), have shown that, in addition to tunability, constant transmissivity and constant bandwidth may be achieved by utilizing a multielectrode Distributed Feedback laser diode configuration. The filter consists of three sections; a phase control section between two active sections with Distributed Feedback. While the gain is controlled by current injection in the active section, the tuning is separately controlled by current injected through the phase control section. Tuning ranges of 120 GHz (9.5 .ANG.) with 24.5 dB constant gain were achieved in this particular case. Furthermore, narrow-band tunable optical filters having a net optical gain have been demonstrated which employ a Distributed Bragg Reflector structure. See Kazovsky et al., ECOC 1989 Proceedings, pp. 25-7. Specifically, Kazovsky et al. utilize a three section structure comprising a phase control section between a Distributed Bragg Reflector and an active section. A resonator is established between the reflector and the interface between the active section and air. In operation, current in the grating section is used to tune the resonant frequency while current in the active section adjusts the optical gain of the filter. As with all the various optical filters described above, the filter exhibits multiple transmission peaks which effectively decreases the tuning range. While Kazovsky et al. suggest that a single transmission peak may be achieved if the length of the phase control section is zero, this is undesirable since tuning would be discontinuous because phase matching between the reflector and the interface could only be achieved at non-continuous wavelengths.
Tunable wavelength optical filters having a wide tuning range which would increase the number of available tunable channels, especially in subscriber loop networks, would be of considerable interest. However, while it is desirable to also minimize the attenuation in the filter, such as by employing an active section, it has been established that the spontaneous emission in the active region generates noise which may have a deleterious effect on its operation. Furthermore, employing an active section causes the filtered output signal to vary nonlinearly with the input signal unless the bias current is adjusted to compensate for such nonlinearity. A related issue is that a low ratio of the filter bandwidth to tuning range limits the number of available channels for a given crosstalk level.